The anaerobic process for the production of biogas (mostly methane and carbon dioxide) is well known. It can be carried out on a continuous basis on waste product streams from a number of industries, typically on aqueous streams including soluble materials derived from plant or animal origin. Such product streams can include suspended solids, to a limited extent; provided they are biodegradable (can be digested in the anaerobic processes). The process can be formally divided into four distinct stages with each stage being carried out by different groups of organisms that in nature can be found together acting in a symbiotic fashion, with the products of one group of organisms being passed on to the next as an energy source (food).
The four distinct phases are:                1. Fermentation and hydrolysis—At this stage the long chain macromolecules present in the effluent solubles stream are broken down to smaller chain molecules. For example in the alcoholic beverages industry there are typically four distinct groups of macromolecule present carbohydrate, protein, oils and fats, and organic acids.        2. Acidogenesis—This involves a group of organisms that produce a broad spectrum of Volatile Fatty Acids (VFA) from the smaller chain substrate produced in (1) above.        3. Acetogenesis—This stage involves the breakdown of VFA to (predominately) acetic acid that becomes the dominant substrate used by a range of methanogenic organisms for the production of methane gas.        4. Methanogenesis—This is the final stage and involves the production of methane gas, together with some carbon dioxide. Typically other products such as ammonia and hydrogen sulphide are also formed.        
As an example the aqueous residue from a fermentation and distillation process as used to produce an alcoholic beverage can undergo hydrolysis and acidogenesis relatively easily (at least after removal of the bulk of the undissolved solids content) as the substrate for the organisms concerned is rich in energy and the ratio of macro nutrients (carbon nitrogen and phosphorus—C:N:P) are in an ideal range for these steps, and subsequent methane production.
However, acetogenesis and methanogenesis have proved to be significantly more difficult steps when attempting to provide an efficient process. An efficient process should result in a good removal of organic content (typically measured as a reduction in chemical oxygen demand—COD) and a corresponding high methane content of the biogas.
For example typical anaerobic digestion processes, such as those employed on waste aqueous effluent in the spirits alcoholic beverages industry may only be expected to operate with a COD removal of the order of the order of 70% or less and produce a biogas having a methane content of 70% or less, with the bulk of the remainder being CO2, and so representing carbon that is not available for energy generation. The methane content of the biogas is highly dependent on substrate composition. Substrates that are rich in carbohydrate will typically have lower biogas methane content. Furthermore, additional processing steps typically including aerobic digestion processing are often required to reduce the COD levels to those acceptable for disposal e.g. to drain.
In U.S. Pat. No. 6,395,173 (Von Nordenskjold—now known as BIOLAK Technology GmbH) an anaerobic treatment apparatus is described that can be used for treatment of “waste waters”. It is suggested the apparatus is suitable for waste waters having a biological oxygen demand (BOD) in excess of 2000 mgl−1. Such a BOD may equate to a COD of about 4000 mgl−1 depending on the source of the organic content and represents a relatively dilute waste stream to be treated. The apparatus includes a two stage sludge bed, of the upflow anaerobic sludge blanket reactor (UASB) type, wherein sludge and liquid effluent from the reactor can be recycled. As is common with waste water treatments an aerobic treatment stage can be applied to the liquid effluent post the anaerobic stage to complete the reduction of COD. A similar option is suggested for dealing with excess sludge. However, waste waters of about 4000 mgl−1 may be considered to be relatively weak and adapting the apparatus and process for digesting waste streams of higher COD concentration is not as straightforward as it may seem. Typically users may consider diluting the liquid waste stream prior to introducing this to the methanogenic apparatus, but such dilution has potentially undesirable consequences in terms of flow rates through the apparatus, hydraulic retention time, maintaining alkalinity and/or conversion rates.
Moreover the waste water may be acidic in pH, particularly when initially subjected to an acidogenesis/acetogenesis step, although many waste waters are naturally acidic. Such acidic pH may have a significant detrimental effect on methanogenesis which is preferably carried out in slightly alkaline conditions. In view of this, many prior art processes control pH through the addition of alkali, such as lime. Saritpongteeraka, K and Chaipratpat, S. (Bioresource Technology 99, (2008) p 8987-8994) describe a process in which pH is controlled through addition of NaOH or parawood ash. This process is carried out in an anaerobic baffled reactor with and without effluent recycle. Their results appear to show pH control is essential, but that effluent recycle has little effect. Nevertheless, the influent COD was typically in the rate of 5000-6500 mg/l and although a long hydraulic retention time was suggested as being of benefit, this was at the expense of the organic loading rate.
Although the above process did not suggest that effluent recycle was of benefit, this is in fact routinely adopted and proposed for use in fluidised bed and UASB reactors treating carbohydrate rich waste waters to make use of alkalinity generated internally and to dilute the influent COD (see Sam-Soon, P., Water S A, 1991; 17(1): p 37-46 and Ferguson J F, Water Research, 1984:18(4) 573-80). Such recycling and its benefits, particularly when employing high COD waste streams is also discussed in U.S. Pat. No. 4,415,453.
It is an object of the present invention to provide methods and apparatus for carrying out an anaerobic digestion of a waste stream that avoids or at least reduces one of the aforementioned problems.